In seismic exploration, particularly for hydrocarbons, it is common practice to impart vibrations to the earth and to provide a plurality of sensing devices (commonly called geophones) to detect reflected vibrations. The recordings of these reflected vibrations provides geologists with information used for predicting the geological formations below the surface and with proper formations, the presence of oil or gas.
Generally, the imparting of such vibrations to the earth has been accomplished by the ignition of dynamite or other explosives. Also, thumpers have been used involving the imparting of a substantial force on a plate on the ground or dropping a plate onto the ground. In exploration for oil and gas formations which occur under water it has been common to create vibrations by sparkers which utilize the discharge of an electrical charge between two underwater electrodes. By being underwater the coupling to the earth has not been a problem but attempts to utilize sparkers on land have met with no success since they have not imparted sufficient energy into the ground to provide useful returned reflection recordings.
Difficulties are also encountered with explosives since they take from two to ten milliseconds to complete their burn. Such long burn time creates point source noise and earth movement in the form of a shear wave which interferes with the initial reflections received by the geophones. In some situations the use of an explosive as the source of vibration imparted to the earth renders recordings of the first fifty milliseconds impractical.